This invention relates to coating materials, and in particular to urethane coating materials such as used as conformal coatings in packaging electronic components.
Polyurethanes are one of the commercially most important classes of synthetic coating materials. They are readily formulated to have excellent physical properties, including high toughness, a controllable range of flexibility, high abrasion resistance, and good moisture and chemical resistance. Other properties such as adhesion and optical transparency can be produced in some types of polyurethanes. The physical properties can be varied over ranges by controlling composition and the reaction conditions of formation.
Polyurethanes also have attractive electrical properties that, combined with the physical properties, make them candidates for use in electrical component packaging applications. Some polyurethanes can be made to be highly stable electrical insulators that do not adversely affect high frequency signals, that do not outgas or otherwise have damaging reaction products, and which adhere to substrates such as circuit boards or chips. Such polyurethanes are prepared in a liquid or flowable state, brushed, sprayed, or poured over the electrical component to be protected, and permitted to harden as a coating over the component. These coatings are called conformal coatings, as they conform to the shape of the component and protect it from external damage.
Urethanes are formed by reacting an isocyanate with a source of active hydroxy groups, to produce a urethane linkage. The use of polyfunctional reactants, those having two or more isocyanate groups and two or more active hydroxy groups, results in the formation of higher molecular weight polymers called polyurethanes. Polyurethanes are preferred for coating applications, and the polyurethane reaction can be accomplished in a number of ways.
In one approach to the polyurethane formation reaction as used to prepare conformal coatings for use in electronics packaging, one component is a prepolymer or adduct having free isocyanate groups, and the other is a resin having reactive hydrogen atoms, such as hydroxy-terminated polyols. These reactants are mixed together in a flowable composition, coated over the component, and allowed to harden. The result is a hard, durable protective coating over the component, which does not interfere with its operation.
The reactants of the polyurethane reaction are usually mixed together prior to the time that they are needed, and stored at low temperature to inhibit the urethane/polymerization reaction. After thawing, the user typically has about 1/2 hour to dilute the mixture with a solvent to render it more flowable, and then to apply the thawed and diluted mixture before the reaction proceeds too far for application to be possible. This procedure is followed for two reasons. First, larger batches can be mixed economically and with proper mixing controls, and then divided and packaged and frozen as individual packets to be thawed as needed. Second, the mixing and freezing is typically accomplished at a central location that has adequate environmental safeguards for preparing the mixtures, which often contain toxic ingredients.
While this approach of preparing ingredient mixtures at a central location, freezing packets of the mixture, and then later thawing and using the packets often works well, there is one significant problem. Even when frozen at -40.degree. C., the mixture deteriorates during storage, so that the shelf life of frozen packets before they must be used is typically only about 1 month. If the packet is not used within about 1 month of being frozen, its quality deteriorates so that it cannot be used and must be discarded. In large scale electronics packaging operations, and those involving critical applications such as spacecraft electronics, there must be careful attention to control and use of the inventory of frozen packets. Where the need for use, and the supply, of conformal coatings are intermittent or periodic, packets of frozen reactants are often discarded after expiration of their specified shelf life. The electronics manufacturer simply cannot risk using deteriorarted conformal coating mixture, because of the possibility of the coating causing failure of the expensive electronics components. Consequently, many thousands of dollars of waste may result each year due to the discarding of packets of frozen reactants that have exceeded their shelf lives.
There is therefore an immediate need for some approach to reducing waste resulting from deterioration of frozen polyurethane coating materials. It is believed that a solution to this problem may also be useful in other fields which use urethanes and polyurethanes as coatings and for other applications. The present invention fulfills this need, and further provides related advantages.